1. Field of the Invention
The present invention relates to termination and connection of mineral insulated (MI) heating units and cables.
2. Description of the Related Technology
Mineral insulated cables are used primarily as heating units and power cables. Those cables have an outer sheathing in the form of a metal tube, one to seven conductors, and insulation of magnesium oxide around the conductors which insulates and also holds the conductors in place inside the metal tube.
Sections of mineral insulated cable may be terminated, joined to each other, or joined to non-mineral insulated cables. For example, a mineral-insulated section of non-heating cable may be joined to a section intended for heating; these two sections might be identical except that the heating section has more-resistive internal conductors.
The conventional method of terminating a mineral insulated cable is to slide an open-ended termination fitting over the outer metallic tube of the mineral insulated cable, fill the interior with mineral insulation similar to that in the cable, and then braze a cap onto the open end of the fitting.
The conventional method of making joints between two sections of mineral insulated cable is to join the conductor wires protruding from the ends of each of the metallic tubes, for electrical continuity, and then complete the joint with a slide-on coupling that is brazed onto the metal tubes. The space inside the coupling is hollow and must be filled. That is conventionally accomplished by drilling a small hole in the side of the coupling, injecting additional mineral insulation to fill the void, and then sealing the hole by brazing. The hole is typically tapped to a 6-32 NC thread into which a mating brass screw is turned. The screw is broken off and the end is brazed over.
This process is both time-consuming and uncertain: time-consuming because the fill hole should, for obvious reasons, be small, and filling the entire void within is a lengthy process; uncertain because it is nearly impossible to assure that the void is evenly filled and packed. Gaps are liable to be left, and any effort to pack the mineral insulation into place might shift the conductors, putting strain on wire joints and possibly even causing a short circuit.
In addition, the mineral insulation is usually very hygroscopic and, inevitably, it absorbs water. The filled-in insulation must then be dried prior to sealing the hole, and that requires great care.
Not only the after-filling, but the brazing of the couplings to the tubes, is likewise time-consuming. Thorough cleaning of the tubes and couplings is needed; all oxide must be removed, or else the joint will not be good.
Brazing involves high temperatures that alter the physical properties of the metal in the tubes, making it brittle and leading to increased liability to cracking and a larger permissible bend radius. It also causes new oxidation, which must be removed. In addition, brazing creates unhealthy fumes.
The prior art does not disclose any method of terminating (capping or joining) sections of mineral-insulated cable which is fast, insures uniform filling of voids between internal conductors, does not require cleaning of oxide or corrosion from metal surfaces, and which does not harm cables through high temperature.
Accordingly, objects of the present invention are to terminate or join sections of mineral insulated cable quickly and reliably; to insure the absence of voids in joined or terminated sections of mineral insulated cable; to eliminate the cleaning of oxide or corrosion from metal surfaces; to keep metal parts below temperatures at which embrittlement occurs; and to avoid unhealthy fumes.
The present invention reduces or eliminates brazing from the processes of joining two sections of mineral insulated cable or of terminating an end of a section of mineral insulated cable. In the present invention, couplings are joined to tubes by crimping, and voids are filled with epoxy. That is faster and more certain than the prior-art methods, does not harm the metal of the tubes, requires a lower level of skill, and eliminates the need for drilling holes in couplings and end fittings.
The present invention may be assembled or made either in a factory or in the field.
With these and other objects, advantages and features of the invention that may become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings attached herein.